Some cyber industry experts and former government investigators suspect that Russians’ newly revealed purchase of $150,000 in election-related Facebook ads was merely a trial run for a much bigger, more secretive operation aimed at helping Donald Trump win the White House.

To discover the truth, the experts say, congressional and Justice Department investigators will need to dig deep, tracing the sponsorship and actual financing of every unique ad that raises suspicions — especially those containing fake news.

The Facebook ads — divulged last week but yet to be made public or shared with congressional probers — were easy to identify because they came from accounts based in Russia.

If Russian operatives disguised additional Facebook advertising by using U.S.-based intermediaries, investigators may only be able to trace the origins of those ads with the Silicon Valley giant’s help.

The disclosed spending was likely detailed “a test buy” in which a Kremlin-connected “troll farm” bought thousands of ads through a maze of phony accounts “to see what works,” said a person with knowledge of Facebook’s operations.

If that’s true, it would be relatively easy for Facebook to search its records and learn who bought ads and whether, as well as how, the ads were targeted, said this person, who insisted upon anonymity to protect relationships.

Robert Mueller, the Justice Department special counsel who is leading criminal and counterintelligence investigations into whether Trump’s presidential campaign coordinated with Russian digital operatives, has resources to try to unravel the financial trail if front companies or nonprofits are discovered to have sponsored suspicious ads.

On Wednesday, Bloomberg News quoted a source who said Russia’s use of social media to spread damaging information about Trump’s Democratic opponent, Hillary Clinton, is a “red hot” focus of Mueller’s inquiries.

Still uncertain is how deeply the House and Senate Intelligence Committees will investigate, given their more modest resources and that the panels are led by Republicans who have shown some reluctance to pursue leads that cast doubt on the election of a GOP president.

North Carolina Sen. Richard Burr, the chairman of the Senate panel, said Tuesday that he wants “a full accounting” from Facebook and other social media companies of any Russian activity during the campaign. He offered no details of what that means.

“We’ve only scratched the surface,” said Mike Carpenter, a former senior Pentagon official who focused on Russia. “In due time, I think we’ll learn of other Russian fronts using Facebook and other social media platforms like Twitter to disseminate politically motivated disinformation.

“It’s also worth remembering that the Kremlin’s disinformation operations subcontract a lot of the less sophisticated propaganda work to surrogates in other countries who are compensated via the Dark Web, making it difficult to follow the money trail,” Carpenter added.

He also said he believes the Russians sought to suppress voting by select groups of Democratic voters who would be expected to vote for Clinton.

Whether the committees will issue sweeping subpoenas to the social media giants could be key in determining how much more information emerges. Because of its pledges of client confidentiality, Facebook may prefer the legal protection of being subpoenaed before surrendering information in a national security investigation.

A Facebook spokesman said Wednesday the company “will continue to investigate and will continue our cooperation with the relevant investigative authorities looking into that subject.”

Google Inc., which owns YouTube, the enormous platform that circulates videos, some of which are paid ads, is “always monitoring for abuse or violations of our policies, and we’ve seen no evidence this type of ad campaign was run on our platforms,” company spokeswoman Andrea Faville said. She declined to elaborate, except to say that the company will cooperate with the investigations.

Besides posting on Facebook, Russian operatives barraged Twitter Inc.’s platform with automated computer commands known as “bots” that carried fake and harshly critical news about Clinton to users across the country. A Twitter spokesman did not respond to a request for comment about the extent to which it is investigating the Russian activity.

The Senate committee’s ranking Democrat, Virginia Sen. Mark Warner, has called Facebook’s disclosure “the tip of the iceberg” and is urging Burr to proceed aggressively. He noted this week that Facebook discovered and shut down nearly 50,000 inauthentic Russian accounts before the French elections in June.

“I believe the Russians were at least as active if not more active in the American elections than they were in the French elections,” said Warner, who previously headed a telecommunications company.

Warner also voiced disappointment with Facebook over its failure, during a briefing for House and Senate Intelligence Committee staffers last week, to disclose that Russian operatives promoted live events such as an anti-immigration gathering sponsored by SecuredBorders in Twin Falls, Idaho, last year. Facebook’s spokesman said the company has removed those Facebook pages.

McClatchy reported in July that both Mueller and the congressional committees are investigating possible collusion between the Trump campaign’s digital operations and Russia’s Trump-slanted cyber meddling.

Jared Kushner, Trump’s son-in-law who oversaw the campaign’s digital operations, is expected to make a second appearance before the Senate committee soon, when he will be questioned by its members, said a person familiar with the matter. Subjects of interest include the possibility that the campaign coordinated with the Russians in helping them target ads to specific voters, said the source, who insisted upon anonymity because the matter is secret.

Facebook’s platform offered a special opportunity to target certain voters — a strategy on which the presidential campaigns spent heavily.

If an ad buyer provides Facebook with a state’s voter registration database or a list of voters who supported either Clinton or Trump, Facebook can match it with people based on their race, views on gun rights or other characteristics, said David Stroup, who ran Warner’s digital operations during his 2014 Senate campaign.

“That’s where some magic happens at Facebook,” he said. “You can tell it to create what they call a look-alike audience.”

“Anyone can post an ad on facebook,” said Michal Kosinski, a Stanford University psychologist who has spent years researching how Facebook data can be used to sway people’s views. “There’s no verification of your name. This basically means it’s virtually impossible to control this space.”

Not everyone is quick to sign on to the view that the Russians’ cyber onslaught was necessarily stealthy.

“There’s a long standing assumption that what Russia did (in the elections) was super sneaky, cunning and well disguised,” said Andrew Weiss, a Russia expert for the National Security Council during the Clinton administration who now is a vice president of the Carnegie Endowment. “What this overlooks is just how noisy and in your face the Russia role actually was.”

Asha Rangappa, a former FBI counterintelligence agent, said Russia was engaging in both covert and overt operations and “they weren’t putting all their eggs in one basket.”

“A foreign intelligence service would likely have needed someone with an intimate knowledge of the political culture in which they’re operating to help guide their actions,” she said. “That would almost certainly be someone who lives here.”

A member of the House Intelligence Committee, Democratic Rep. Eric Swalwell of California, recalled U.S. intelligence agencies’ assessment last January that Russia is reviewing what lessons it has learned and almost assuredly will do this again.

‘”We have to assume they’re sharpening their knives and they’re going to come at us again,” he said.